It says refugees trying to cross into neighbouring Ingushetia are now commonly subjected to full body searches.

There are unconfirmed reports that Chechen men of military age have been detained at the border and taken to Russian military headquarters at Mozdok.

Last week the Russians banned all men from the age of 10 to 60 from crossing the border. The ban was later lifted after international pressure.

More than 2,000 Chechens, mostly women and children, arrived in neighbouring Ingushetia on Thursday, the highest daily figure in weeks.

The UN agency estimates up to 180,000 Chechens remain in the neighbouring republic.

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BBC correspondent Claire Doole says that with most escape routes blocked and refugees coming under heavy artillery fire, few are willing to try to escape.

Describing Grozny as "an active battle field", the UN says only a handful of people have left the capital in the past few weeks.

It estimates 20,000 Chechens are still trapped there, hiding in cellars without electricity or adequate food and water.

Both sides are suffering losses in battles around two locations close to the centre.

Russian military officials say fighting is fierce for control of a strategic bridge which leads to the centre and for a square not far from the city centre which was the scene of heavy clashes during the last war.

Commanders say the Chechens are moving along tunnels to try to cut off and destroy Russian positions and to break through Russian lines encircling Grozny.

They say rebels made one such attempt last night but were driven back.

The Russians say 80 rebels have been killed since Thursday against the loss of eight federal troops.

Heavy losses

Reports that the Russians are sustaining far heavier losses are impossible to verify as commanders are not letting foreign journalists anywhere near the zone of combat.

But observers say the sheer weight of Russian numbers and military hardware makes it all but inevitable that they will eventually take Grozny.

They have just appointed a military commandant to oversee the establishment of a provisional civilian administration in the expectation that it will be days, rather than weeks, before he starts work.

Meanwhile, Russian Defence Minister Igor Sergeyev said on Friday that he knew nothing about the fate of the general who went missing in action on Tuesday.

He also announced that Nato Secretary General George Robertson would visit Moscow next month in the first such trip since Russia froze its ties with the organisation over the Kosovo crisis.

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